The present invention generally relates to communication systems, and more particularly, to a system and method for peak to average power ratio reduction in communication systems using time domain techniques for data signal transmission. The present invention is particularly suited to transmitters that use pulse amplitude modulation (PAM) and Tomlinson preceding.
In recent years, telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, control signals, PSTN (public switched telephone network) information, as well as, information from ancillary equipment in analog form (i.e. computer modems and facsimile machines) that is transmitted in the POTS bandwidth.
Prompted largely by the desire of businesses to reliably transfer information over a broadband network, telecommunications service providers have employed DSL systems to provide a plethora of interactive multi-media digital signals over the same existing twisted-pair copper lines. The provision of asynchronous digital subscriber line (ADSL) systems, using discrete multi-tone (DMT) line coding, to customer premises has proliferated over recent years due to the increasing demand for high speed Internet access.
ADSL systems can be designed to operate over the same copper lines with either the POTS or a Basic Access Integrated Service Digital Network (ISDN-BA) service. ADSL systems are designed to transmit significantly more data in the downstream direction, that is, from the telecommunication service provider to the remote customer, rather than sharing the bandwidth as in a dual-duplex communication system. The ADSL configuration permits the concurrent transmission of multiple digital signals from the telecommunications service provider to a remote location, while providing an adequate bandwidth for the limited capacity required for data transmissions originating at the remote site.
High-Bit-Rate digital subscriber lines (HDSL) can be considered an extension of the DSL and ADSL systems. HDSL was developed as a dual-duplex repeaterless T1 technology. HDSL was designed to serve non-load telephone subscriber loops meeting the Carrier Serving Area (CSA) guidelines. Unlike ADSL systems, HDSL systems are designed to take advantage of the full data transmission capacity of a cable pair in both directions. It is important to note that ADSL systems use DMT modulation for data transmission. Conversely, HDSL systems use PAM, a broadband carrierless method, for data transmission.
In a basic HDSL DMT implementation, a HDSL transmission unitxe2x80x94central office (HTU-C) is configured to modulate and convert digital data for analog transmission from a central office downstream to a remotely located HDSL transmission unitxe2x80x94remote (HTU-R). Each end of the HDSL link configured at the end points of a CSA loop is provided a similarly configured HDSL transmission unit. It is important to note that the central office and remote designators for the two HDSL transmission units is for descriptive purposes only as both units are interchangeable with the remote (HTU-R) unit configured to receive the analog transmission sent from the HTU-C from the telephone line. The HTU-R demodulates the signals and applies error correction before delivering each of the reconstructed digital data signals to its intended device. Concurrently, the HTU-R transmits data from the remote location back to the HTU-C.
In HDSL, incoming data bits are first encoded in PAM symbols, then subjected to Tomlinson preceding to better match the exact characteristics of the line. The encoded and modified PAM symbols are then passed through a bandwidth shaping filter.
Constraints on the average transmitted power in communication systems vary according to the specific application. In DSL applications, constraints are imposed to limit the amount of interference, or crosstalk, radiated into neighboring receivers. Because crosstalk is frequency dependent, the constraint on average power may take the form of a spectral mask that specifies the magnitude of allowable transmitted power as a function of frequency. For example, crosstalk in POTS communication systems is generally caused by capacitive coupling and increases as a function of frequency. Consequently, to reduce the amount of crosstalk generated at a particular transmitter, the pulse shaping filter generally attenuates higher frequencies more than lower frequencies.
In addition to constraints on the average transmitted power, a peak power constraint is often imposed as well. This peak power constraint is important for the following reasons:
1. The dynamic range of the transmitter is limited by its line driver and its digital to analog converter (DAC). In particular, saturation of the line driver will xe2x80x9cclipxe2x80x9d the transmitted waveform. Similarly, signal magnitudes that exceed the dynamic range of the DAC will also xe2x80x9cclipxe2x80x9d the transmitted waveform.
2. Rapid fades can severely distort signals with high peak to average power.
3. The transmitted signal may be subject to other non-clipping induced non-linearities.
4. A major component of the total power consumption within a transmitter is consumed within the line driver, as line driver power consumption overwhelms the power consumed by digital signal processing. As a result, PAR reduction is important to decrease power consumption in the output line driver.
The peak to average power ratio is defined as the ratio of the magnitude of the instantaneous peak power to the time averaged value of the power. In HDSL2 applications, the PAR is defined by the digital transmit shaping filter. The shaping filter is added in the modulation stage within the HTUs to shape the spectrum of the transmitted signal. The effect of a digital finite impulse response (FIR), often used as the shaping filter, can be analyzed by calculating the mean, the variance, and the peak value of the FIR filter output using random data as the FIR filter input. It can be shown that the worst case PAR is uniquely defined by the filter coefficients, Hi, as                     PAR        =                              Σ            ⁢                          "LeftBracketingBar"                              H                i                            "RightBracketingBar"                                                          Σ              ⁢                              xe2x80x83                            ⁢                              H                i                2                                                                        Eq        .                  xe2x80x83                ⁢        1            
This theoretical maximum is in fact almost never achieved, so it is typical to define the PAR at a given level of significance by defining an effective peak value Pe(s). Pe(s) is such that the probability that the absolute value of the voltage exceeds Pe is 10xe2x88x925. It is also usual to present the negative cumulative probability of the absolute voltage Q(v), so that Q(Pe(s))=10xe2x88x925. Once Pe(s) is known, PAR(s) is simply Pe(s)/RMS.
Two frequency domain techniques have been proposed to reduce PAR in transmitted signals in communications systems, see e.g. J. Tellado and J. M. Cioffi. xe2x80x9cFurther Results on Peak-to-Average Ratio Reductionxe2x80x9d, ANSI T1E1.4 contribution number 98-252, San Antonio, Tex., Aug. 1998 and ETSI/ANSI TM6 contribution number TD15, Vienna, Austria, Sep. 21-25, 1998. The proposed techniques are repeated for each frame, or in other words, for each IFFT operation. The techniques may be summarized as follows:
1. Reserving a set of tones whose amplitudes are calculated either by linear programming or a simpler iterative procedure to minimize the PAR. It is significant to note that this proposal reduces the data rate which is unacceptable under HDSL communication standards.
2. Modifying the modulation scheme by permitting more than the minimal set of points in each QAM constellation. Typically, a constellation is replicated at integer multiples of a translation vector, so that one input data symbol may be mapped at different positions in the QAM plane. Under this proposal, constellation positions are selected to minimize the PAR either via an iterative or a linear programming procedure. It is significant to note that this proposal does not reduce the data rate, but results in an increase in the average power dissipated in the line driver.
Neither of the aforementioned frequency domain techniques is of much practical interest as linear programming solutions require excessive resources and the proposed iterative solutions would necessitate undesirable time delays. In addition, the first technique does not apply for broadband carrierless modulation schemes.
Minimizing transmitter power for a given data rate in a given bandwidth is a recurrent theme in designing communication systems. As such, a number of techniques have been developed. However, to date, PAR has been adjusted by modifying the modulation (or encoding) technique or by selecting a specific shaping filter.
Accordingly, it is desired to provide a system and method that efficiently, accurately, and timely reduces the peak to average power required in a transmission unit in a communications system using a broadband carrierless data transmission scheme.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention provides a system and method for reducing the peak to average power in a transmitter designed to use a broadband carrierless modulation scheme. Briefly described, in architecture, the system can be implemented by introducing a comparator in a feedback loop of the causal portion of a digital shaping filter and introducing a correction signal in the feedback loop when the magnitude of encoded symbols exceeds a predetermined threshold. The corrected encoded signals may be processed by both the causal and residual portions of the digital shaping filter prior to digital to analog conversion. The system of the present invention is particularly suited to systems that use Tomlinson preceding.
In systems that use the Tomlinson preceding method for receiver correction, the PAR reducer may be inserted to further modify the encoded symbols prior to digital to analog conversion. A prediction may be made of the peak values that would be obtained if the original output of the Tomlinson precoder was proceesed by the shaping filter. If the absolute value of the predicted peak value exceeds a threshold, a correction of a full 2L step may be applied for one sample of the Tomlinson precoded symbol stream. The correction step is applied in such a way as to reduce the resulting peak output.
Because the receiver also uses a mod 2L demapper, no change is needed at the decoder in the receiver. As such, the PAR reducer of the present invention may be implemented as a transmitter only solution.
Two methods of predicting the peak values are presented, one splits the filter into causal and non-causal filter segments so that no extra delay is introduced in the process, the second method uses a two-pass approach where a first pass gives the exact values without correction, then the corrected values are injected in a duplicate modulation filter (this implies an extra delay equal to the filter delay). Note that an N-pass approach may be derived from the two-pass approach.
A variation of the two-pass approach takes advantage of the linearity of the system to estimate the correction. Since the modulation scheme is not completely linear, the variation of the two-pass implementation is an approximation of the other embodiments.